Position control apparatus



Sept. 10, 1935. D. J. STEWART v POSITION CONTROL APPARATUS l0Sheets-Sheet 1 Original Filed Aug. 20, 1930 INVENT-OR P lgmcamcJISf/ew'art 6W WMQ ATTORNEYS iiiiii Sept. 10, 1935. D. J. STEWARTPOSITION CONTROL APPARATUS Driginal Filed Aug. 20, 1930 10 Sheets-Sheet2 INVENTOR J. Stewart;

ATTORNEYS P 1935-- D. J. STEWART POSITION CONTROL APPARATUS OriginalFiled Aug. 20, 1930 10 Sheets-Sheet 3 INVENTOR Duncan J. StewartATTORNEYS Sept. 10,1935." D. J. STEWART POSITION CONTROL APPARATUSOriginal Filed Aug. 20, 1932) 10 Sheets-Sheet'4 WMM ATTORNEYS Duncan cl' fitewart- I I Sept. 10, 1935. D. J. STEWART POS ITION CONTROLAPPARATUS 1o Sheets-Sheet 5 HLx t W w m W 5 O w m In NJ 9 Z V w m t w wm 4 w mm/ 4 c WW1! 7 .D

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ATTORNEYS Sept. 10, 1935. DJ. STEWART 2,013,676

I POSITION CONTROL APPARATUS Original Filed Aug. 20, 1930 l0Sheets-Sheet 6 Fig. Z

INVENTOR Duncan J Stewart ATTORNEYS P D. J. STEVWART I 2,013,676

POS ITION CONTROL APPARATUS l Y 4 l] INVENERSt 75' 5 rrw n...r uncan warWMMM' ATTORN EYS s lo, 1935..

D. J. STEWART POS ITION CONTROL APPARATUS 10 Shets-Sheet 8 OriginalFiled Aug. 20, 1950 dq V y 6 n h m u av W n 5 z n 5 m 6 ,6 u i H O J J aJ J c WM a 0 WW M)/ m w w lil 7 J J 6 L N. .H V

lNVENTOR gunoan J. Stewart mm MM,

' ATTORNEYS I Sept. 10, 1935.. D. J. STEWART POSITION CONTROL APPARATUSOriginal Filed Aug. 20, 1933 10 Sheets-Sheet 9 INVENTOR gyncan clSiuuarfi ATTORNEYS Sejat. 10, 1935.. D. .1. STEWART 2,013,676

POS ITION CONTROL APPARATUS Original Filed Aug. 20, 1930 10 Sheets-Sheet10 TO M GT0 RS /67 I69INVENTOR gyncan J. Steward ATTORNEYS enema Sept.10, less POSITEON CQN'EROL APPARATUS Duncan .5. Stewart, Rockiord,lIilL. assignor to liiowardD. Colman, Rockford, ill.

Application August 20, 1931?? Serial No. 476,557 Renewed November 3t,1934 3 3. illaims. (or. sac-13.4)

This invention relates to a system for controlling the position of oneobject in accordance with the position of another object and has moreparticular reference to the automatic control of the movements ofcutting tools and the like in cam cutting, die-sinking or other machinesfor shaping the surface of-a work-blank to conform to a. pattern.

The general object of the invention is to provide a novel mechanism ofthe above class by which a higher degree of accuracy than has beenpossible heretofore may be obtained in the positiomng of a controlledobject to correspond to the position of a controlling object.

Another object of the invention is to provide a new and improved meansfor. detecting with ex treme accuracy variations in positional relationof a cutting tool and a pattern controlled object and controlling, inaccordance with such variations, a power driving means for efiectingrelative feeding movements between the tool and a work blank so as tomaintain the positional relation between the tool and controlling objectsubstantially constant while the surface of the work blank is beingtraversed.

Minute variations in electrical impedance such, for example, as thecapacity of a variable condenser having closely spaced plates may bemeasured with a high degree of accuracy. Therefore, in carrying out theforegoing object, an electrical impedance is gradually and constantlyvaried in response to the v movement of a pattern controlled object suchas a tracer and also according to the movement of a cutting tool and a.work blank relative to each other, and the variations in impedanceabove and below a predetermined value are utilized to control the di-,

rection of feed between the tool and work blank so that the impedanceand therefore the positional relation of the tracer and tool aremaintained within a narrow range of variation.

Another object of the invention is to provide a novel electrical meansfor detecting the variations in the reactance of the device abovementioned and controlling the direction and extent of tool feed in'accordance with such variations.

Still another object is toprovide a novel means for conditioning theelectrical means preparatory to a contouring operation.

Other objects and advantages of the invention will become apparent fromthe following detailed" description taken in connection with theaccompanying drawings, in which,

Figure 1 is a front elevational view of a me- 55 of Fig. 1.

' to limit the invention to the specific form dis- 30 chine toolembodying the features of the present invention.

Fig. 2 is a side elevation of the machine looking from the left in Fig.1.

Fig. 3 is a plan view of the machine; 5 Fig. 4 is asectional view takenalong the line 5-5 of Fig. 2.

Fig. 5 is a sectional view takenalong the line Figs. 6 and '1 aresectional views taken respecl0 tively along the lines E6 and ll-E ofFig. 2.

Fig. 8 is a sectional view t ken along the line 83 of Fig. 7.

Fig. 9 is a sectional view taken along the line 9-9 of Fig. 3.

Figs. 10 and 11 are sectional views taken respectively along the-linesIll-'40 and li-H of Fig. 9.

' Figs. 12, 13 and 14 are sectional views taken respectively along thelines |2i2,'l3l3 and 20 l t-Hi of Fig. 2. E

Fig. 15 is a schematic view and wiring diagram of the principal parts ofthe machine .and the controls therefor.

While the invention is susceptible of various 25 modifications andalternative constructions, I have shown in the drawings and will hereindescribe in detail the preferred embodiment, but it is to be understoodthat I do not thereby intend closed, but intend to cover allmodifications and alternative constructions falling within the spiritand scope of the invention as expressed in the appended claims.

35 The mventzon m general The invention is especially adapted for use inthe automatic control of a power driving mechanism for efiectingrelative movements between a controlled object, such for example as acutting tool, and a work blank to be operated thereon. In the presentinstance, the control is utilized in a machine for cutting cam surfacesembodying a tool I adapted to be fed relative to a work: blank 2 inopposite directions along a single fixed path according to variations inthe surface of a pattern 3, which variations are detected by acontrolling object such as a feeler 4.

The movement of the controlling object and also the relative movementbetween the tool and work blank are communicated to an electrical devicehaving a variable impedance which changes gradually and automatically inresponse to changes in the positional relation of the controlling andcontrolledobjects. In the present instance, the device is a condenser 5(Figs. 7, 8 and 15) having closely spaced plates, and the variations inthe capacity reactance above and below a predetermined value aredetected by means including a thermionic tube and utilized throughappropriate electromagnetic means to control selectively the directionof feed of the tool by a power driving means. The direction of such feedis always such that the change in the condenser capacity accompanyingthe change in the position of the tool opposes the capacity change dueto the feeler 4, with the result that the spacing of the condenserplates and therefore the condenser capacity remains approximately atsaid predetermined value and within an extremely narrow range ofvariation therefrom. In this way, a predetermined positional relationbetween the feeler and the tool is' accu rately and automaticallymaintained while different portions of the pattern surface are beingpresented to the feeler and corresponding portions of the Work blank arepresented to the tool.

While the present system of control is adapted for use withvarious typesof cutting tools, it is preferred to employ a cutter of the rotary typewhich may be fed into and out of the work to produce cuts of differentdepthssimply by shifting the cutter axis laterally.

Support for the work "blank and pattern and advance thereof which ispressed into and thereby made rigid with a driving spindle in theform ofa sleeve 1 having a flanged end to which the master cam disk 3 isdetachably but rigidly secured. The

spindle projects through and is journaled in the lower end of a pedestal8 upstanding from a carriage 9 (Fig. 2) mounted to slidehorizontallyalong ways Ill and II and supported from the machine bed I 2 by a kneel3 which is slidable along vertical ways l4. Adjustment of the carriagealong the different ways to set the pattern and blank initially for themilling operation may be effected by manual rotation of the usual screwshafts l5, l6 and ll (Fig. 1).

Power for rotating the spindle I is supplied by an individual electricmotor |8 supported by the carriage through the medium of averticallyadjustable bracket l9 and acting through the medium of a gearing 20,cone pulleys 2|, a belt 22, a planetary gearing 23 (Fig. 4), to drive ashaft 24 which carries a worm 25 meshing with a worm wheel 26 fast onthe end of the spindle sleeve 1 opposite the master cam. With thisgearing, the spindle is rotated at a very slow speed which may be variedwithin the desired limits by shifting the belt 22. The blank and mastercam may, when desired, be rotated by hand by turning a wheel 21 fast onthe shaft 24.

Mounting and drive for the tool Herein the tool I is rotary millingcutter whose axis is disposed parallel to and is shiftable laterallytoward and away from the axis of the work so that the cutter may be fedinto and out of the periphery of the work at the same time thatsuccessive sections of the periphery are being presented to the cutter.To support the cutter for such feeding movement, its tapered shank 28(Fig. 9) is received in one end of a 5 spindle sleeve 29 journaled atspaced points in a tool slide in the form of a tubular head 30 locatedeccentrically with respect to the axis of a supporting cradle in-theform of a hollow drum 3| mounted in the bed of the machine in end- 10to-end relation with respect to the work spindle The drum 3| is mountedfor a limited degree of oscillation about a fixed axis spaced from andextending parallel to the work axis so that angular movement of the drumin opposite directions 15 will carry the cutter toward and away from thework axis and thereby vary the depth of the cut made by the tool. Tothis end, the peripheral surface of the drum is journaled in an annularbearing 32 (Fig. 9) supported by a casing 33 20 through the medium oftwo threaded rings 34. The cooperating surfaces of the drum and bear ingare tapered so that looseness in the drum mounting may be taken up byturning the rings 34 and thereby shifting the bearing axially. 25 Inorder that the eccentricity of the cutter relative to the cradle axismay. be varied, the head 30 is constructed at opposite ends withrectangularly-shaped flanges 35 which are slidable 'in radial guideways36 formed by plates 31 which 30 are rigid with opposite ends of the drum3|. A

screw shaft 38 (Fig. 9)v rigid at one end with the head 30, threads intoa rotatable sleeve 39' journaled in the drum casing and carrying ahandwheel 40. By turning the latter in opposite di- 35 rections, thehead may be moved radially of the drum any desired distance as indicatedby a scale 4|. When the head is in engagement with a stop 42 6), theaxis of the cutter coincides with that of the drum. 40

With the cutter thus mounted, it will be apparent that angular movementof the drum 3| will feed the cutter toward or away from the axis of thework in a fixed arcuate path determined by the radial spacing of thehead relative to the drum axis.

The cutter spindle 29 is driven constantly from an individual'source ofmotive power which comprises an electric motor 43 (Fig. 3) supported onthe machine base and having its shaft 44 con- 50' nected through a geartrain 45 (Fig. 13) with the end of the spindle 29 opposite the cutter.The gears of the train are rotatably supported within a housing 46 whichis pivoted at one end to the tool slide 30. The other end of the housingis pivotally connected to one end of a link 41 (Figs. 3 and 13) whoseotherend is pivoted on the machine base to swing about an axiscoincident with the motor shaft. This arrangement provides for rotationof the cutter from the stationarily mount- 0 ed motor iri all of thedifferent positions of the cutter. r

Toolfeed cutter constantly in one direction or the other 75 but throughvarying distances governed by the contour of the pattern surface.

As best shown in Fig. 12, the reversing mechanism herein employedincludes two magnetic friction clutches having oppositely rotatingdriving elements in the form of sleeves 49 and 58 arranged in end-to-endrelation and mounted in ballbearings supported by a bracket plate 5|upstanding from the top of the machine base and, spaced from the drum3|. The adjacent ends of the sleeves are formed with flanges whoseopposing surfaces are adapted for frictional gripping engagement withopposite sides of a disk 52 fast on a shaft 53 which constitutes thecommon driven element of the two clutches. This shaft extends axiallythrough the sleeves, and is supported thereby through the medium ofball-bearings which permit of slight axial movement so that the disk 52may be carried into engagement with either of the driving surfaces. Thesleeves 89 and 50 are driven at high speed from the motor 38 by means ofa single endless belt 54 which passes successively around a pulley 55fast on the motor shaft 55, a pulley 5'! fast on the sleeve 49, an idlerpulley 5B, and a pulley 59 keyed to the sleeve 50.

Axial shifting of the shaft 53 to cause engagement of the two clutchesselectively is controlled by two electromagnets 60 and GI whoseringshaped cores 62 have friction faces disposed on opposite sides of anarmature in the form of a disk 63 of magnetic material rotatable on butheld against axial movement relative to an extended end portion of theshaft 53. For this purpose, the hub 68 of the disk is supported throughballbearings by two disks 55 rigid with the shaft 53 and having conicalbearing surfaces. With this arrangementjt will be apparent that themagnet 68 will, when energized, attract the armature causing the drivenclutch disk 52 to engage the sleeve 49 while reverse rotation ofthe-shaft will occur upon energization of the magnet (ii. Theelectromagnets provide for the required sensitivity of control for thepower actuated cutter drive while the employment of clutches of thefriction type causes immediate response of the reversing mechanism tothe selective magnetic control.

Rotary motion of the shaft 53 is transmitted to a shaft 66 (Fig. 12)through the medium of intermeshing pinions- 61 which permit of the axialmovement of the shaft required to operate the two clutches. Within ahousing 58, the shaft'66 carries a worm 69 meshing with a worm-wheel 10which is rigid with a sleeve ll rotatably supported by a non-rotatableshaft 12. The sleeve H constitutes the driver of a planetary gearinggener ally designated by the numeral 13 (Fig. 11) and rotatable about agear M fast on the shaft 12. This gearing drives a sleeve 15 alsorotatable on the shaft 12 and constituting the driver of a secondplanetary gearing 16 which rotates about a stationary gear I8. Thedriven gear 11 of the second planetary system is fast on a shaft 19 andprojecting parallel to the axis of the drum 3|. This shaft carries apinion 88 meshing with gear teeth formed around the outer periphery ofthe end plate 31 of the drum. A loose gear 8| meshing with the teeth onthe drum is also carried by the shaft 19. An adjustable spring 82 (Fig.10)

acting in torsion between the gear 8| and the shaft 19 serves to take upany back-lash in the connection between the shaft and the drum.-

The worm gearing and the two planetary gears constitute the speedreducing system above referred to and are so constructed that thereducrection to hold the blades in their bearings.

tion in the motor speed is approximately in the ratio of 70,000 to 1.With this large reduction, it will be apparent that a large number ofrevolutions of the motor shaft will be required to cause a perceptiblefeeding motion of the cutter. 5

By turning a crank fitted onto the outwardly projecting end of the shaft66 (Fig. 12), the drum 30 maybe moved by hand to position the cutterproperly preparatory to the cam cutting operation. This position may hegaged from a scale 10 83 (Fig. 6) marked on a peripheral surface of theend plate 91. To look the drum and therefore the cutter i in anypredetermined position while the blank is being brought into properposition, an arm 85 pivoted on a stationary part of the 15 machine maybe swung from a normally inactive position to the full linepositionshown in Fig. 9 into the space between adjacent teeth of thedrum gear when the tool slide is in truly vertical position.

Condenser construction and mounting The condenser 5, which is used inthe present machine for detecting minute variations in the relativepositions of the master cam tracer t and 2 the cutter, comprises twometallic plates and 86 (Figs; 7 and 8) having opposed parallel surfacesof equal areas machined and polished to a high degree of flatness. Airis employed as the dielectric to permit relative approaching and re- 30ceding movement of the condenser elements. The condenser is housedwithin a closed casing. to exclude dust and other particles from thesurfaces which are always slightly removed from or disposed just out ofcontacting engagement with 35 each other, being spaced apartapproximately .001 inch in the present instance. With the plates thusclosely spaced, the movement of one or the other plate will produce alarge and readof the condenser. In orderto lighten the associated. partsand facilitate the maintenance of their surfaces in true parallelplanes, the plates preferably are disposed in horizontal planes.

One of the plates may be mounted for bodily 45 movement perpendicularto. its flat surface in response to the movements of the tracer and alsoof the cutter or, as in the present instance, both plates may bearranged for movement, one by the tracer and the other by the cutter.both plates are disposed within a housing formed in part by a plate 81to which the plate 85 is secured but separated therefrom by a sheet ofinsulating material.

The lower. condenser plate 86 is mounted with 55 the housing for limitedmotion relative to the upper plate and is guided in such movement bymeans capable of maintaining exact parallelism of the plate surfaces.Outwardly projecting arms 88 and 89 rigid with the plate 86 are adaptedto 60 rest on ledges '90 projecting inwardly from a depending flange 9|of the plate 81. and serving to limit the movement of the plate 86 afwayfrom the upper plate. At one side of the plate 86 is a knife blade 92slightly inclined relative to the horizon.- tal with its opposite endshaving bearing in'V- shaped grooves 93, The opposite side of the lowerplate is guided by a blade 94 extending parallel to the blade 92 andhaving bearing in grooves formed in the arm 89 and in a lug 95 dependingfrom the plate 81. A spring 96 urges the plate 86 in a. di-

The blades thus constitute a parallel motion a mechanism which maintainsthe opposite sides of 7'5 To this end, 50

the plates 85 and equally spaced during their limited motion relative toeach other.

The lower end of the condenser housing is completely closed by a plate91 secured at its edges to the flange 9I and having a central aperturein which two flexible metal diaphragms 98 are supported. A threadedfeeler pin 99 projects through and is secured by nuts I00 to thediaphragms with its upper end adapted for bearing engagement with thecentral portion of the lower hardened surface of the condenser plate 86.Thus the pin 99 is freely movable in an axial direction to raise andlower the plate 86 and thereby decrease and increase the capacity of thecondenser. When the pin is unsupported by external means, the lowerplate 86 rests on the ledges 90.

Tracer control of condenser In the present instance, the feeler ortracer element 4 above generally referred to comprises an elongatedrigid bar mounted in longitudinal alinement with the feeler pin 99 forreciprocation in a vertical path intersecting the axis of the master cam3. Hereinthe bar is slidable in ways I M (Figs. 5 and 6) defined by arigid extension of the pedestal 8, the lower end of which extension isformed with slots I02 to receive the upper peripheral portionof themaster cam disk when the latter is positoined as previously described.

The lower end of the bar 4 is bifurcated and carries across-pin on whichis rotatably mounted an anti-friction roller I03 which rests upon andfollows the contour of the master cam surface, the bar 4 sliding up anddown in its guideway as the Also mounted on the cross-pin on oppositesides of the roller I03 are rollers I04 which bear against I the sidesof the guideway' IN and thereby maintain the roller I03 accuratelypositioned. Rigid with the upper end of the feeler bar 4 is a flattopped table I05 carrying a hardened metal disk I06 upon which the lowerpointed end of the pin 99 bears so that the vertical motion of the baris communicated to the lower condenser plate. With the feeler 4 thusmounted and associated with the condenser, there is at all times auniform load on the master cam surface which avoids the introduction oferrors due to the spring of the parts.

Cutter control of the condense The oscillatory feeding movements of thecutter I about the axis of the drum 3| are converted into reciprocatorymotion which is applied to the upper condenser plate in a direction tomove the plate toward and from the lower plate 86 in response tomovements of .the cutter respectively toward and from the work axis. Tothis end, the condenser housing is attachedto the downwardly hanging endof a flexible metal tape I01 whose other end follows around thecircumference of a segmental drum I08 which is oscillated by the cutterthrough a connection with the cradle 3 I. At its lower end, the tape I01is attached to a bar I09 (Fig. 7) projecting into a yoke IIO secured toa casing III which in turn encloses and is made rigid with the condenserhousing plate 81. The yoke issupported through a ball-bearing II2resting on a flange I I3 at the lower end of the bar I09 and permittingthe work carriage, the trader, and the condenser to be swung throughanangle of ninety degrees as is required to adapt the present machine forthe cutting of barrel cams.

The drum sector I08 is pivoted on a horizontal shaft H4 supported at oneend in abearing II5 which is supported by the upper end of the pedestal8 and has a detachable connection therewith permitting the bearing toremain stationary during pivoting of the work carriage 9 for the purposeabove mentioned. The other end of the shaft H4 is mounted in a bearingII 6 rigid with the horizontal arm of an L-shaped bracket II'I (Fig. 6)which is adapted to be made rigid with but vertically adjustable along apedestal I I8 upstanding from the machine base and spaced laterally fromthe pedestal 8.

The tape I0I.follows a substantial length of the j sector surface and issecured to the drum sector through the medium of a clamp I I9 slidablealong a bracket I20 on the sector by a micrometer adjusting meansincluding a thumb nut I2 I. With the aid of the clamp and the nut theeffective length of tape and therefore the position of the condenser boxmay be varied with the required accuracy. The radius and center ofcurvature of the drum sector are such that the downwardly hanging end ofthe tape is tangent to the drum surface for all angular positions of thesector. In this way, the oscillatory movement of the drum sector isconverted into reciprocatory motion of the straight end of the tape andthe upper condenser plate.

In the present instance, oscillatory movement of the cutter cradle istransmitted to the drum sector I08 through the medium of a substantiallyrigid rod I 22 to the lower end of which is secured a sleeve clamp I23pivoted at I24 to the end plate 31 of the drum 3|. At its upper end, therod I22 carries an adjustable clamp I25 which is pivoted at I26 to thefree end of an arm I21 rigid with the hub of the drum sector I08. Thepivots I24 and I26 are spacedequidistant from the axes of the drums 3|and I08 so that when the effective length of the link I22 is adjusted toequal the distance between the drum axes, a parallelogram linkage isformed which oscillates the drums in unison through equal angulardistances.

Assuming that a fixed distance is maintained between the condenserplates by oscillating the cutter cradle, it will be apparent that theabove linkage connection provides for movement of the pivot I24 througharcuate distances exactly equal to that of the master cam tracer. Thecutter, being carried by the drum, moves corresponding distances, theratio of the cutter movements to those .of the pivot I24 beingdetermined by the relative radial positions of the pivot and cutteraxis. The relation of the rises and falls on the master pattern to thoseonthe cam to be formed are. thus determined. Q1

\ 55 From the foregoing, it will be apparent that the closely spacedcondenser plates are arranged to follow each other upwardly anddownwardly according to the pattern contour and during such movement inunison are adapted for a limited range of relative movement toward andaway from each other, which range is less than .0001 inch in the presentinstance. During such relative movement, the plates are maintained inaccurate alinement with each other which is accomplished in the presentinstance by a knifeblade guiding means. This means comprises anelongated horizontal arm I28 (Fig. 6) rigid with the housing I II andhaving at its outer end a horizontal notch I29 receiving a blade I30 onthe end of a counterbalance arm I3I integral with the table I05. Thisconnection effectually maintains the two condenser plates in exactparallelism so that the capacity of the condenser is solely a functionof the spacing of the plates.

aoracve In the present instance, the condenser 5 is utilized as avariable impedance and variations in its value relative to apredetermined mean value are detected by impressing an alternatingcurrent voltage across this variable impedance and a fixed impedance inseries relation and then impressing the voltage drop across the fixedimpedance upon the input circuit of an electric discharge device orthermionic tube whose output circuit controls the selective energizationof the magnets 60 and 6 I. Herein the fixed impedance is in the form ofa resistance element I32 (Fig. 15) one terminal of which is connected bya conductor I33 to one insulated blade I30 of a double pole-double-throwswitch I35. When the blade is thrown to the left as viewed in Fig. 15,the conductor I33 is joined to a conductor I36 leading to a binding postI31 rigid with the insulated condenser plate 85 and projecting throughthe casing III.

Alternating current from supply conductors I38 is applied to the twoinsulated} terminals of the condenser and the resistance element throughthe medium of a potentiometer I30 by which the voltage may be adjustedto the desired value and read on a voltmeter I40. connected across theoutput terminals of the potentiometer. One of these terminals isconnected by a conductor III to one terminal of the resistance elementI32. The other terminal is joined to the other blade, I42 of the ER: ER

. (2X3.1416fPA) Where E is a voltage applied to the condenser andresistance element of R ohms, is the alternating current frequency, A isthe area of each condenser plate, P is the dielectric constant and b isthe distance between the plates. When all of these factors excepting bare. constants, it will,

be apparent that separation of the plates will produce a decrease in thevoltage across the resistance and a decrease in b will increase thisvoltage.

To detect variations in this voltage drop, the latter is impressed uponthe input circuit of a three element vacuum tube I 84 with the resultthat the current in the output circuit of the tube decreases andincreases as the condenser plates are moved away and toward each otherrespectively. For this purpose, one terminal of the resistance I32 isconnected by a conductor I45 to the filament I56 of the tube which isheated by current from a battery other terminal of the resistance isconnected to the grid I50 through the medium of a switch I5I and abattery I52 which serves to bias the grid I01 and flowing through aswitch M8 and a rheostat I09. The

In order to amplify the current in the output circuit of the tube I00,the voltage drop across the resistance I51 is impressed between the gridI59 and filament I of a second vacuum tube I6I by a conductor I62connecting one terminal of the resistance II to the filament I60 and aconductor I88, switch I60, 9. battery I65 connecting the other terminalto the grid I59. The filament I60 is heated by a battery I56 under thecontrol of a rheostat I 61. The battery I65 serves to bias the grid ofthe tube I6I. By proper inter-adjustment of the biasing potentials ofthe bat; teries I52 and IE5, the sensitivity, that is the change in theplate current in the tube I6! for a given change in the capacity of thecondenser 5, maybe made a maximum. The'output circuit of the amplifyingtube I6I is supplied with current by a battery I61 and extends from thefilament I60 through a conductor I88, battery I61, switch I60, thewinding I of an electromagnetic relay HI, a conductor I12, an ammeterI13, to the plate I of the tube I6I.

With the circuit arrangement above described, it will be apparent thatthe winding I10 is constantly energized and that an increase in thevoltage drop across the resistance element I32, caused by movement ofthe condenser plates closer to each other, will result in a progressiveincrease in the current in the output circuits of the tubes I04 and I6Iand therefore in the winding I10. On the other hand, the currentenergizing the relay will decrease progressively as the condenser platesmove apart.

The intensity of the energizing current determines the position of therelay armature I15 through the medium of which the current variationsare utilized to control the selective operation of switches no and mwhich govern the opening and closing of thecircuits through the magneticclutch windings 60 and 6| which in turn control the reversing mechanismto deter mine the direction in which the cutter is fed relative tothe-work and the direction of movement of the upper condenser plate 85.

The magnetic clutch circuits controlled by the switches I16 and I11 arearranged to be energized from a source of direct current, one terminalof which is connected by a conductor I18 toa te'rminal common to the twoswitches I16 and I11. The other terminals of the switches are connectedrespectively by conductors I19 and I80 to terminals of adouble-pole-double-throw switch I8I. When the actuator I62 of the latterswitch is thrown downwardly as viewed in Fig. 15, the conductors I19 andI80 are connected to conductors the influence of its spring I80 whichoccurs when the current energizing the'winding I10 falls below apredetermined value in response to a separation of the condensers by adistance of more than .001 of an inch. Such energization of the wind ingcauses the cutter to be fed into the work and thereby'make a deeper out.In this move ment of the cutter the condenser plates are brought closertogether progressively, thereby in-- creasing the current in the relayI10. By the time the plates have approached to within approximately.0009 inch of each other, the ourgized upon movement of the armature I15under spaced .0009 inch apart.

control circuits have been properly conditioned rent in the relay willhave increased to a value sufiicient to overcome the influence of thespring I89 whereupon the armature will be drawn in a direction to closethe switch I16 which results in energization of the clutch winding 6|.This reverses the direction of the feed and the cutter is again movedout of the work and the condenser plates Separated. The result is thatthe cutter oscillates back and forth relative to the axis of the workblank through a range not exceeding .0001 inch, which range is, ofcourse, determined by the sensitivity of the control circuits.

Conditioning 0 the control circuits Means is provided for enabling theoperator to test the condition of the operating circuits above describedand to adjust the plate currents and the grid biasing potentials foroptimum sensitivity, that is, maximum current variation in the outputcircuit of the tube I (il in response to a given change in the condensercapacity. This means includes a condenser I89 connected to two terminalsof the switch I35 so that it may be interposed in series relation withthe resistance element I32 and thereby substituted for the condenser 5when the actuator of the switch I35 is thrown to the right as viewed inFig. 15. The capacity of the condenser I86 is fixed at a value justequal to the mean fixed value of the condenser 5, that is, the valuewhen the plates and 86 are spaced approximately .001 inch apart.

A second condenser I81 is adapted to be thrown in parallel with thecondenser I86 upon closure of a switch I88 and the value of its capacityis such that the combined capacity of the two fixed condensers is justequal to the capacity of the condenser 5 when the plates of the latterare Closing and opening the switch I88 thus has the same effect on thecontrol circuits as varying the spacing of the plates 85 and 86 from.001 inch to .0009 inch and vice versa. The grid biasing potentialsproduced by the batteries I52 and IE5 are then adjusted so as to give amaximum variation in the relay current as indicated on the ammeter I13when the switch I88 is opened and closed. Then the contacts of theswitches I16 and I11 and the tension of the armature spring I89 areadjusted to give the most positive opening and closure of the switchesI16 and I11 in response to the current variation produced bymanipulation of theswitch I88.

Manual control of cutter movements The double-throw switch I8I isprovided'to enable the windings 60 and BI to be energized selectivelyunder manual control so that the operator may cause the'cutter I to movein one direction or the other under the power of the driving motor 48.To condition the circuits for such control, the switch actuator I82 isthrown upwardly so as to connect the conductors I19 and I80 toconductors I90 and I9! leading respectively to terminals of switches I92and I93 which have a common actuator I 94 connected by a conductor I95to the power conductor I18.

Operation The manner of setting the difierent parts of the machinepreparatory to milling an open-face type of cam adapted for use with afollower which is adapted to rock about a fixed axis, and the operationof the machine during the milling will now be described, it beingassumed that the with the aid of the test condensers I86 and I81.

With the master cam 3 and the roughly shaped work blank 2 rigidlymounted on their supporting spindles and the table 9 shifted to withdrawthe blank axially out of cutting relation with respect to the tool, theshaft 66 (Fig. 12) and the wheel 40 are turned by hand to bring the axesof the cutter and the drum 3I into exact vertical alinement and toseparate the axes by a distance exactly equal to the spacing of theroller and pivotal axes in the rocking follower with which the cam is tobe used. Next, the table 9 is raised or lowered to bring the work andcutter axes to the same vertical level. moved horizontally a distancejust equal tothe mean radius of the cam to be formed at the highestpoint thereon plus half the diameter of the follower roller with whichthe cam is to be used.

After locking the table in the position above described with the tool incutting relation to 'the work, the work spindle 1 is turned by thehandwheel 21 until the feeler roller I03 contacts the highest point onthe master cam surface. Then the parallelogram linkage is set and lockedin exact parallel relation after which the drum is rotated by the motor48 by manipulation of the manually operable switch lever I82 until thecutter just clears the work blank at the maximum diameter of the latter.

Next the work table is shifted forwardly which the tape is firmlylocked. Finally the motor I8 is started and the operation allowed tocontinue automatically for one revolution of the work blank.

Assuming now that when the cutter feed is Then the work table is Thenthe tape placed within the control of the condenser, the

winding 60 of the reversing mechanism becomes energized which meansthatthe condenser plates are spaced more than .001 inch apart. Thiscauses the relay winding I10 to be energized by current of an intensityinsuflicient to overcome the tendency of the relay spring I89. Theswitch I11 is thereby maintained closed. Energization of the winding 60initiates movement of the toolsupporting cradle 3| in a direction tocarry the cutter toward the work axis and the condenser plate 85 towardthe plate 86. Such approach of the condenser plates causes a gradualdecrease in the capacity reactance of the condenser and a correspondingincrease in the current energizing the relay winding I10.

When the spacing of the condenser plates falls below .0009 inch and thecapacity thereby rises above the mean range to which the circuits areset to respond, the force of the armature spring I89 will be overcome bythe attractive force of the electromagnet I10 whereupon the switch I11will be opened and the switch I16'closed. En-- master cam surface.

aeiaeve to again close. The resulting energization oi the clutch winding60 reverses the direction of the cutter feed and also the change in thecondenser capacity. i

From the foregoing, it will be apparent'that the capacity of thecondenser 5 is constantly increasing or decreasing according to thedirection of motion of the cutter relative to the work. Thus, while thetracer roller we is traversing a true dwell surface of the mastercam,the capacity of the condenser will be governed solely by themovements of the cutter and the latter will oscillate intermittentlytoward and from the .work axis. The range of such oscillation isdetermined by the sensitivity of the circuits for measuring thecondenser capacity. With one stage of current amplification and thecondenser plates spaced approximately ;001 inch apart, the presentcircuit arrangement will respond to cutter movements of less than .0001inch.

The cutter also oscillates back and forth while the tracer isencountering a rise or fall of the This follows from the fact that. themaster cam is always rotated at such a slow speed that the rate at whichthe tracer moves in response to arise or fall in the master surfacebeing traversed is less than the rate at which the cutter may be fedtoward or from the work axis. ,Thus, when the tracer encounters a fallin the master cam, the intervals during which the cutter is fed into thework are longer than those during which outward movement of the cutteroccurs.

It will be observed that in so far as the feeding movement of the cutterrelative to the, work blank is due to changes in the position of thecondenser plate which is actuated from the master cam, such movement isalways in a direction to oppose the change in the condenser capacityproduced by the master cam. That is to say, the cutter end, therefore,the plate 85 are fed in a direction to maintain a fixed spacerelationship between the condenser plates 85 and 86 and therefore apredetermined mean value of the condenser capacity. In this action, theplate 86 tends to follow the plate 85 with the result that the movementof the cutter, as it travels around the periphery of the work blank,accurately conforms to the contour of the pattern surface.

The roughing cut of the cam is completed in one revolution of the workblank. Then, by ad-. justing the nut l2l of the micrometer clamp N9, thecondenser control may be conditioned for the finishing cut of the camsurface which is accomplished in another revolution of the work.

Safety control To obviate the danger of disfiguring a partially cut camor damaging the machine in the event that the condenser plates are movedrelative to each other beyond their normal range, means is providedwhich operates automatically to render the cutter and work blank feeddrives ineffectual. In the present instance, the trouble is indicated bya sensitive feeler which is capable of detecting either the approach ofthe plates beyonda predetermined point toward each other or separationof the plates beyond a predetermined. point, both of which points beingoutside of the normal range of relative movement between the plates.

The sensitive feeler herein employed is in the form of a lever I91(Figs. 3, Sand 15) swingable vertically about a fulcrum pin I98supported by is disposed between the opposed ends "of the screws I98.Thus, the relative movement between the condenser plates is communicatedto and multiplied by the lever I97.

The outer end of the elongated arm of the lever is disposed betweentwoadjustable contacts 203 and cooperates therewith to form two switches204 and 205 which are connected in parallel with each other and inseries with a winding 206 of an electromagnetic switch 201. By properadjustment of the screws I99 and 204, the feeler may be so set that theswitch 204 will be closed when the plates 85 and 86 separate more than apredetermined distance and the switch 285 will be closed when the platesapproach too close to each other. Both points of closure of the switchesare beyond the normal range of relative movement between the condenser,plates so that both switches will ordinarily be open and will so remainas long as all of the machine is functioning properly.

Closure of either of the switches 208 and 205, causes the winding 285 tobe energized from a source of current thereby attracting an armature 208which opens the switch 20'? causing deenergization of an electromagneticrelay 209 whose armature is thereby allowed to move in a direction toopen switches 2! and 2H which control the operation of the work andcutter feed motors I8 and 48. In this way the milling operation issuspended automatically before damage is done to the work or to themachine parts.

' I claim as my invention:

1. In a machine for reproducing on a work blank the contour of a patternsurface, the combination of a work support, a tool support, powerdriving means, two electromagnetically controlled clutches operableselectively to cause feeding movement of one of said supports inopposite directions by said power means, a condenser comprising twoopposed plates adapted for movement toward and away from'each other, oneof said plates being movable in unison with said movable support, acontrol element adapted to trace said pattern surface automatically andto move said other condenser plate in accordance with changes in thecontour of said surface, means operating to detect variations in thecapacity of said condenser including an electromagnet having an armaturemovable in opposite directions respectively as the condenser capacityrises above or falls below a predetermined value, and .a pair ofswitches selectively controlling the energization of said clutches-andeach adapted to be closed and opened respectively in the movement ofsaid Lil means controlling said reversing mechanism and operable inresponse to an increase in capacity above a predetermined value to causesaid tool to be advanced in a direction to cause a decrease in saidcapacity, said last mentioned means acting in response to a decrease inthe capacity below a predetermined value to cause reverse movement ofsaid tool and an increase in the condenser capacity.

3. In a machine for controlling the feeding of a tool relative to a workblank so as to reproduce on the latter a surface conforming to thecontour of a pattern, said machine combining a tracer element' arrangedto follow the pattern surface, a common means supporting said patternand blank in operative association with said element and toolrespectively, means supporting said tool for feeding movement into andout of said blank, power-driven means for moving said first mentionedsupporting means to present successive portions of said pattern to saidracer element and corresponding portions of said blank to said .tool,power driven means for moving said tool supporting means, a condensercomprising, two closely spaced plates connected respectively to saidelement and tool and adapted for movement in unison while said tool andtracer element are in a predetermined positional relation andfor'iimited relative movement according to variations in such relation,and means responsive to changes in capacity of said condenser andcontrolling said last mentioned power driven means to cause the toolsupporting means to be fed in directions such as to maintain asubstantially uniform spacing of said condenser plates regardless of theposition of the condenser plate controlled from said tracer.

4. In a machine for controlling the feed of a tool to reproduce on awork blank the contour of a pattern surface, said machine combiningmeans supporting the pattern and said blank for rotation in unison abouta common axis, a feeler contacting a peripheral surface of said patternand adapted during the rotation of the pattern to move in oppositedirections along a fixed path according to variations in the surfacecontour, means supporting said tool for movement to different depthsinto and out of a peripheral surface of said blank, reversible powerdriving means for feeling said tool supporting means, a condenser thecapacity of which varies solely as a function of the spacing of itsplates, means for varying the condenser capacity in accordance with themovements both of said feeler and said tool supporting means, and meansresponsive to changes in the condenser capacity and controlling saidpower driving nieans to cause said tool supporting means to move in adirection to oppose a change in the condenser capacity produced bymovement of said feeler.

5. In a machine for controlling the feed of a cutting tool to reproduceon a work blank the relative feeding movements in difierent directionsbetween said tool and work blank, and means responsive to changes in thecapacity of said corrdenser and controlling the direction of operationof said power driving means to maintain the condenser capacity atsubstantially constant value.

6. In a machine for controlling the feed of a cutting tool to reproduceon a work blank the contour of a pattern surface, a feeler arranged totrace the pattern surface automatically, a condenser having its platesmechanically associated with said feeler and said tool so that thespacing of the plates varies with changes in the relative positions ofsaid tool and feeler, and power actuated means acting in response tochanges in the condenser capacity to feed said tool varying distancesinopposite directions so as to maintain a substantially uniform spacing ofthe condenser plates while said feeler is tracing the pattern surfaceand said tool is being presented to different portions of said workblank.

7. In a machine having a work support and a tool support mounted forrelative feeding movement therebetween, the combination of means formoving one of said supports relative to the other including a reversingdevice having an electromagnetic control means, a pattern controlledelement, a condenser having relatively movable plates connectedrespectively to said element and said movable support, and meansresponsive to changes in the capacity of said condenser for governingthe energization of said electromagnetic means.

a 8. In a machine for reproducing a pattern surface on a work blank, thecombination of a work support, a support carrying a cutting tool, acondenser comprising two closely spaced plates, means for impartingrelative feeding movements between said supports to one of said plateswhereby to vary the condenser capacity, means for varying the spacing ofthe condenser plates continuously according to variations in thecontour. of the pattern surface, power driving means for moving one ofsaid supports in opposite directions relative to the other support, andmeans responsive to the capacity of said condenser and controlling thedirection of movement of the movable support by said power driving meansso as to maintain said plates slightly removed from contactingengagement witheach other,

9. In a machine for reproducing on a work blank the contour of apattern, the combination of a combined work and pattern support, amovable'tool support, a feeler adapted to trace a surface of saidpattern, a condenser whose capacity varies with changes in thepositional relation between said feeler and tool support, meansreincluding .anele'ctromagnet having an armature arranged to move inopposite directions when the condenser capacity rises above and fallsbelow a predetermined capacity range, power means for feeding said toolsupport in opposite directions, and a reversing device governing thedirection of such feed and arranged to be controlled selectively by saidarmature.

10. A pattern reproducing machine having, in combination, a cutter, apattern tracer, and means including a variable capacity condenser forcontrolling the feed of said cutter to reproduce the movements of saidtracer.

11. A pattern reproducing machine combining a cutter, power actuatedmechanism for feeding said cutter continuously in one of a plurality ofdifferent directions, a pattern-controlled member, a condenser thecapacity of which is varied continuously in accordance with changes inthe positional relation of said cutter and member, and detecting meansresponsive to such capacity e,o1s,c7c

changes to govern the direction of the cutter feed ments will efiect arelatively large variation in the condenser capacity, and meansresponsive to the capacity of the condenser and acting to control thedirection of the feed by said mechanism.

13. In a machine for controlling the feed of a cutting tool to reproduceon a work blank the contour of a pattern surface, a feeler arranged totrace the pattern surface automatically, a pair a I of closely spacedcontrol elements mounted. for

broadwise approaching and receding movement relative to each other andhaving opposed edges defining a narrow elongated gap, said elementsbeingconnected respectively to said feeler and tool so that the area of saidgap varies with changes in the positional relationship of said elements,and power actuated means responsive to changes in the area of said gapand acting to feed continuously said tool in one direction when the areaof said gap exceeds a predetermined value and in a difierent directionwhen the area is less than a predetermined value.

14. Ina machine for controlling .the feed of a cutting tool 'toreproduce on a work blank the contour of a pattern surface, a feelerarranged to trace the pattern surface automatically, a pair of closelyspaced control elements mounted for sidewise approaching and recedingmovement relative to each other in response to changes in the positionalrelation between said feeler and tool, the opposing edges of saidelementsdefinlng a narrow elongated slit between them, and poweractuated means acting in response to variations in the relative spacingof said elements-to feed said tool constantly in one of a plurality ofdiiferent directions whereby to maintain a substantially uniform spacingof said elements and therefore agreement in the positional relation ofsaid feeler and tool.

15. In a pattern reproducing machine, the combination of a poweractuated mechanism for effecting relative feeding movements between acutter and a work blank, a control device including two elements havingtwo closely spaced parallel edges defining a narrow elongated slitbetween them, means for varying the spacing of said elements accordingto a pattern contour, and means responsive to variations in the spacingof said elements to control continuously the direction of feed by saidmechanism.

16. In a machine for reproducing the contour of a pattern, thecombination of power actuated feeding means for efiecting relativemovement between a tool and a work blank to vary the depth ofthe cutmade by the former, a control device comprising two elements thepositional relation of which is varied automatically in accordance withvariations in the pattern contour and also with changes in the relationof said tool and work blank, an electromagnet arranged to be energizedconstantly by current which varies progressively with changes in thepositional .relation of said elements, and means controlled by saidmagnet for governing the operation of said feeding means in onedirection when the current intensity increases above a predeterminedvalue and in a difierent direction when the current falls below apredetermined value. I

17. In a machine for reproducing a pattern, the combination of a traceradapted to follow. the

pattern contour, power actuated feeding means for effecting relativemovement between a tool and a work blank to vary the depth of cut madeby the former, power actuated means for causing successlve sections ofsaid pattern and work blank to be presented to said tracer and toolindependently of the relation of the tool and work blank, a

pair of control elements the positional relation of which is varieddiiferentially according to the relation of said tool and blank andalsoaccording to the relation of said tracer and pattern, a thermionicdevice having an input circuit controlled from said elements so thatthe-current in its output circuit fluctuates progressively with changesin the positional relation of the elements, an electromagnet arranged tobe variably energized in accordance with said current fluctuations, andmeans controlled by said electromagnet governing the operation of saidpower feeding means to determine the direction of the relative motionbetween said tool and work blank while said tracer is traversing thepattern surface.

18. In a machine for reproducing a pattern 2 contour on a work blank,the combination of a work support, a support carrying a cutting tool, anelectrical device having a reactance which varies with relative movementbetween said supports, means arranged to trace the pattern surface andto cause said reactance to be varied progressively in accordance withthe contour thereof, means for detecting changes in said reactanceincluding a constantly energized electromagnet, power actuated means forfeeding'one of said supports in difierent directions, and meansresponsive to the degree of energization of said magnet for controllingthe direction of movement of said movable support by said power means.

19. In a machine for reproducing a pattern contour on a work blank, thecombination of a work support, a' support carrying a cutting tool,

an electrical device having a reactance which means for said power meansresponsive to changes in said reactance and acting to cause the movablesupport ,to be advanced in a direction to oppose the change in reactanceproduced ynder the control of said pattern.

20. In a machine for maintaining a predetermined positional relationbetween a cutting tool and a pattern controlled element 'while thelatter is tracing the pattern surface, the combination of a condenserthe capacity of which is varied progressively in accordance with changesin the positions of said tool'and element, an electrical resistanceconnected in series relation with said condenser, means impressing analternating current voltage across said condenser and resistance,

. a thermionic device having the voltage drop across saidresistanceimpressed upon its input circuit, an electromagnet adapted to bevariably energized in response to current fluctuations in the outputcircuit of said device, and power driving means for causing relativefeeding movements between said tool and a work blank, and a reversingdevicecontrolling the direction of such feed in response to saidelectromagnet.

21. In a machine for governing the relative feeding movements between acutting tool and a device and said element, an electromagnet, means.

acting in response to fluctuations in the voltage across said resistanceelement'to produce corresponding fluctuations of unidirectional currentin said electromagnet, and power driving means for causing said feedingmovements controlled by said electromagnet. 22. In a machine forgoverning the relative feeding movements between a cutting tool and awork blank in accordance with the contour of a pattern surface, thecombination of a condenser the capacity of which is varied with saidfeeding movements and in accordance with the contour of said pattern, afixed resistance element in series with said condenser, means impressingan alternating current voltage across said condenser and said elementjathermionic device having input and output circuits, the voltagefluctuations across said resistance element produced by variations inthe spacing of the condenser plates being impressed upon said inputcircuit, means for amplifying the unidirectional current fluctuation insaid output circuit, an electromagnet arranged to be variably energizedby the amplified 'current,

and power means controlled according to the. degree of energization ofsaid el'ectromagnet for feeding said tool and work blank relative toeach 1 other in opposite directions.

23. In a machine for controlling the feed of a cutting tool to reproducea pattern on a work blank, the combination of a pair of closely spacedmetallic plates constituting a condenser having a predetermined-meancapacity which isvaried automatically according to the contour of apattern surface and to changes in the relative positions of said tooland work blank, power driven means for controlling the relation of saidtol and said blank, a fixed resistance connected in series with saidcondenser, means impressing an alternating current voltage across saidcondenser and resistance, a thermionic device having the voltage dropacross said resistance impressed upon its input circuit, anelectromagnet selectively controlling said power driven means responsiveto current fluctuations in the output circuitof said tube, a secondcondenser having a fixed capacity equivalent to the mean value of saidvariable condenser and adapted to be substituted for the latter inseries with said resistance while the circuits for saidtube are beingconditioned, and a third condenser of relatively lower capacity adaptedto be thrown into and out of parallel relation with respect to saidsecondcondenser to produce an effect simulating a minute variation inthe spacing of the variable condenser plates.'

24. In a machine for controlling the feed of a cutting tool to reproducea pattern on a work blank, the combination of a pair of closely spacedmetallic plates constituting a condenser having a predetermined meancapacity which is varied automatically according to the contour of apattern surface and to changes in the relative positions of said tooland work blank, power driven means for controlling the relation of saidtool and said blank, means for detecting minute variations in thecapacity of said condenser including a thermionic device having inputand output circuits, power driven means for efiecting relative feedingmovement between said tool and blank, electromagnetic means controllingthe direction of such feed in response to current fluctu ations in saidoutput circuit, and means for permitting sensitive adjustment of thecircuits for said device including a second condenser having a fixedvalue corresponding to the mean value of said variable condenser.

25. In a machine for reproducing the contour of a pattern, thecombination of a control device responsive to changes in the positionalrelation of a cutting tool and a pattern controlled object, a thermionicdevice having its input circuit controlled by said control device, poweractuated means for effecting relative feeding movements between saidtool and a work blank, means controlled by current fluctuations in theoutput circuit of said thermionic device for determining the directionof feed of said tool by said power actuated means, and a test device ofa fixed character corresponding to a predetermined relation of saidtooland said object, said test device being adapted to besubstituted forsaid control device during conditioning of the circuits of saidthermionic device.

26. In a machine for reproducing the contour of a pattern, thecombination of a control device responsive to changes in the positionalrelation of a cutting tool and a pattern controlled object, a thermionicdevice having its input circuit controlled by said control device, poweractuated meansfor effecting relative feeding movements between said tooland a work blank, means controlled by current fluctuations in the outputciroperated so as to produce in said thermionic device an effectcorresponding to a predetermined variation in the positional relation ofsaid tool and said object.

27. In a machine for varying the positional relation of a controlledobject in accordance with changes in the position of a controllingobject, the combination of means for effecting movement of saidcontrolled object in one direction or the other, a condenser thecapacity of which is varied automatically and progressively with changesin the positions of said objects, and electromagnetic means responsiveto the value of said capacity and controlling said power actuated meansto reverse the direction of movement of said controlled object when thevalue of said capacity rises above or falls below a predetermined value.

28. The combination with an object to be moved to different positionsaccording to the position of a controlling object, of power drivenmechanism for moving said controlled object, a conchanged. to apredetermined value.

29. Mechanism for causing a controlled object to follow the movements ofa controlling ob- "ject comprising closely spaced control elementsmovable relative to each other with changes in the position of each ofsaid objects, and means

